The epidermis of aerial plant tissues is covered by the hydrophobic cuticle, a two-component material consisting of plant waxes and the polymer cutin. The polyester cutin is composed of esterified .omega.-hydroxyfatty acids (Kolattukudy, P. E., Espelie, K. E., and Soliday, C. L. 1981. Hydrophobic layers attached to cell walls. Cutin, suberin and associated waxes. Pages 225-254 in: Plant Carbohydrates II--Extracelluar Carbohydrates. W. Tanner and F. A. Loewus, eds. Springer Verlag, Berlin).
Cutinases are defined as esterases with cutin-hydrolytic activities (Koller, W. 1991. The plant cuticle: A first barrier to be overcome by fungal plant pathogens. Pages 219-246 in: The Fungal Spore and Disease Initiation in Plants and Animals. G. T. Cole and H. C. Hoch, eds. Plenum Press, New York). Mostly produced by microorganisms, cutinases are involved in the saprophytic degradation of leaf cuticles by soil organisms (Stahl, D. J., and Schaafer, W. 1992. Cutinase is not required for fungal pathogenicity on pea. Plant Cell 4; 621-629, Koller, W., Yao, C., Trail, F., and Parker, D. M. 1995. Role of cutinases in the invasion of plants. Can. J. Bot. 73 (Suppl.1): S1109-S1118.) and in the penetration of plant cuticles during early steps of plant infection, primarily by plant pathogenic fungi (Koller, W. 1991. The plant cuticle: A first barrier to be overcome by fungal plant pathogens. Pages 219-246 in: The Fungal Spore and Disease Initiation in Plants and Animals. G. T. Cole and H. C. Hoch, eds. Plenum Press, New York; Koller, W., Parker, D. M., and Becker, C. M. 1991. Role of cutinase in the penetration of apple leaves by Venturia inaequalis. Phytopathology 81: 1375-1379).
The enzymatic activity of cutinases is not restricted to cutin hydrolysis. Cutinases have been shown to act as lipases, and some lipases also hydrolyze cutin (Martinez, C., Nicolas, A., van Tilbeurgh, H., Egloff, M.-P., Cudrey, C., Verger, R., and Cambillau, C. 1994. Cutinase, a lipolytic enzyme with a preformed oxyanion hole. Biochemistry 33, 83-89; Gerard, H. C., Fett, W. F., Osman, S. F., and Moreau, R. A. 1993. Evaluation of cutinase activity of various industrial lipases. Biotechnol. Appl. Biochem 17: 181-189).
Cutinases and lipases belong to the enzyme class of esterases (Koller, W., and Kolattukudy, P. E. 1982. Mechanism of action of cutinase: Chemical modification of the catalytic triad characteristic for serine hydrolases. Biochemistry 21: 3083-3090; Derewenda, Z. S., and Sharp, A. M. 1993. News from the interface: The molecular structures of triacylglyceride lipases. Trends Biochem. Sci 18: 20-25), together with the large family of non-specific caboxylesterases with broad substrate specificities (e.g.; Aida, K., Moore, R., and Negishi, M. 1993. Cloning and nucleotide sequence of a novel male-predominant carboxylesterase in mouse liver. Biochim. Biophys. Acta 1174: 72-74). All esterases including cutinases, lipases and non-specific carboxylesterases hydrolyze chromogenic model esters such as acyl esters of p-nitrophenol (Huggins, C., and Lapides, J. 1947. Chromogenic substrates. IV. Acyl esters of p-nitrophenol as substrates for the colorimetric determination of esterase. J. Biol. Chem. 170: 467-482).
Esterases such as lipase and cutinase have been shown to improve the biological effect of fungicides and other pesticides by decomposing the waxy layers (=cuticles) of plants (Iwasaki, T., and Hioki, Y. 1988. Enhancement of biocide. U.S. Pat. No. 4,762,547).
The sole activity of esterases as inducers of plant defense reactions and, therefore, as sole agents for plant disease control with a mode of action not relating to an enhancement of fungicide efficacies has never been described. Utilization of esterases for the control of plant diseases would provide a non-toxic and environmentally benign means of plant protection and, thus, would be of great importance to agriculture.